The HLA complex is critical for host recognition of organ grafts and for recognition of foreign antigens in general. Subsets of T-lymphocytes recognize class I and class II antigens, directly in the case of allogeneic responses and in association with foreign antigens in normal immune responses. The objectives are to determine the role of intracellular interactions between HLA glycoproteins and internalized antigens in generating immune responses, and to understand the specific aspects of HLA biosynthesis and intracellular transport which facilitate these interactions. Intracellular class II HLA complexes, containing Alpha-subunits, Beta-subunits, the lectrophoretically invariant (I) chain and an associated proteoglycan, will be isolated and characterized. The mechanisms and sites of assembly and disassembly of these complexes will be investigated. The role of the I-chain and proteoglycan in determining the temporal parameters of intracellular transport of class II antigens will be determined. As models for internalized antigens, conjugates of transferrin and Fab'2 fragments of rabbit anti human immunoglobulin with neuraminidase and other enzymes will be prepared. Intracellular interactions between these ligand-enzyme conjugates and HLA antigen in B-lymphoblastoic cell lines (B-LCL) will be studied. These ligands will also be conjugated to peroxidase, bound to B-LCL, and endosomes containing the conjugate will be purified and examined for their content of newly-synthesized HLA antigens and other membrane glycoproteins. Similar approaches will be used to study intracellular interactions of HLA antigens with influenza virus components. The capacity of B-LCL-derived endosomes containing internalized flu virus to elicit in vitro secondary T-cell proliferative responses will be investigated.